This invention relates to a method of making a spark-plug centre electrode of the type comprising a composite body made up of a sheath part in the form of a hollow body of corrosion-resisting material which is open at one end, and a core, made of a material having good thermal conductivity, which is inserted into the hollow interior of the sheath part and is jointly shaped with the sheath part by plastic shaping to obtain the resultant composite body.
In German OS No. 22 38 283, there is described a method of producing spark-plug centre electrodes, etc. in which method a piece of metal in convex form and having good thermal conductivity, e.g. copper, is introduced into a piece of metal of corrosion-resisting material such as nickel, which is of concave form matching the shape of the other metal piece, and the assembly is plastically shaped by extrusion to provide the final form. The starting point for this method of production is formed by a drawable material which consists of a cup-shaped corrosion-resisting metal piece with a thermally conducting core inserted therein, and it is assumed that, if required, the two metal pieces can be joined together by brazing, welding, heat-diffusion, compression, calking, etc. The purpose of this joint is to produce a drawable material which can be readily handled and, after the two parts have been suitably joined, can be easily introduced into a cold-drawing die. The drawable material is then converted into rod form by cold drawing under pressure applied by a ram, so that a cold-drawn product is obtained that comprises a cap-shaped sheath part of corrosion-resisting material which surrounds a metal core having good thermal conductivity.
When testing spark-plug centre electrodes consisting of a sheath part and a core, it has been found that overheating causes rapid destruction of the centre electrode if a satisfactory specific transfer of heat is not possible between the sheath part and the heat conducting core, particularly in the zone of greatest generation of heat. A likewise known form of spark-plug centre electrode of this kind therefore comprises a brazed joint between the sheath part and the core, which joint is also extended to the zone of the inner basal surface of the sheath part. However, such brazed joints can be produced only with difficulty in closed-off cavities to give the required uniformity, and a brazed joint which is not free from pores and is not perfectly uniform over the entire zone of the inner basal surface leads to a build up of heat and therefore to rapid destruction of the centre electrode.